1. Field of the Invention
The present invention relates to a water-absorbing resin widely used in the sanitary material field such as disposable diapers and sanitary napkins, the agriculture and forestry field, the civil engineering field, and the like, and to a method for producing the same. In particular, the present invention relates to a water-absorbing resin for the sanitary material field such as disposable diapers and sanitary napkins requiring a high water absorption capacity, and to a method for producing the same. More particularly, the present invention relates to a water-absorbing resin excellent in the water absorption capacity under pressure and under no pressure, and to a method for producing the same.
2. Description of the Related Art
In recent years, water-absorbing resins which gelate by absorbing large amounts of water are developed as one of synthetic polymers and widely used in the sanitary material field such as disposable diapers and sanitary napkins, the agriculture and forestry field, the civil engineering field, and the like. Many kinds of water-absorbing resins are known as these water-absorbing resins, including, for example, a crosslinked partially neutralized polyacrylic acid (for example, refer to Patent Document 1), a hydrolysate of a starch-acrylonitrile graft polymer (for example, refer to Patent Document 2), a neutralized product of a starch-acrylic acid graft polymer (for example, refer to Patent Document 3), a saponified product of a vinyl acetate-acrylate copolymer (for example, refer to Patent Document 4), and a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer (for example, refer to Patent Document 5).
The properties that the above-described water-absorbing resins should have include a high water absorption capacity and an excellent absorption rate when these resins are brought into contact with aqueous liquid such as body fluid, liquid permeability, gel strength of a swelled gel, the absorbing amount to absorb water from a substrate containing an aqueous liquid, and the like. However, the relationship between these properties does not necessarily show a positive correlation, but physical properties, such as liquid permeability, gel strength, and absorption rate, tend to be reduced as the water absorption capacity increases.
A technique to crosslink the vicinity of the surface of a water-absorbing resin is known as a method for improving various water absorption properties of these water-absorbing resins while keeping them in balance, and various versions of the technique have been proposed so far.
For example, various methods in which the following crosslinking agents are used therein are known, including a method of using a polyhydric alcohol (for example, refer to Patent Documents 6 and 7), a method of using a polyvalent glycidyl compound, a polyvalent aziridine compound, a polyvalent amine compound, or a polyvalent isocyanate compound (for example, refer to Patent Document 8), a method of using a glyoxal (for example, refer to Patent Document 9), a method of using a polyvalent metal (for example, refer to Patent Documents 10 and 11), and a method of using a silane coupling agent (for example, refer to Patent Documents 12, 13, and 14).
Further, attempts to perform a uniform surface crosslinking by coating a crosslinking agent on a surface of a water-absorbing resin during crosslinking reaction are known, including a method in which an inert inorganic powder is present (for example, refer to Patent Documents 15 and 16), a method in which a dihydric alcohol is present (for example, refer to Patent Document 17), a method in which water and an ether compound are present (for example, refer to Patent Document 18), and a method in which phosphoric acid is present (for example, refer to Patent Document 19), when the crosslinking agent is added.
Furthermore, as an application of the above-described methods, a method is also known in which, for the purpose of reducing a residual monomer, a neutralized monomer having a specific composition is mixed with a crosslinking agent as described above and the surface crosslinking is performed during the heat treatment of the resulting mixture (for example, refer to Patent Documents 20, 21, and 22). There is also disclosed a method for producing a water-absorbing resin excellent in the water absorption rate and water absorption capacity under pressure by conducting polymerization in the presence of a blowing agent and subjecting the resulting polymer to surface crosslinking treatment (for example, refer to Patent Document 23).
However, although the balance of various properties of the water-absorbing resin is improved by these methods, it is not yet sufficient, and further improvement is required. In particular, when the property requirement of the water-absorbing resin used in the absorber for sanitary articles having a reduced thickness by using a large amount of water-absorbing resin, which represent a notable trend in recent years, is taken into account, the current state of affairs is that the conventional methods as described above have not yet provided such resins with a sufficient level of physical properties.
Moreover, the demand of disposable diapers for elderly people is increasing as the increase of average life in recent years. For a disposable diaper for elderly people, a water-absorbing resin having “an excellent absorption ratio under high loads so that the diaper can make full use of the absorption capability even when heavier loads are applied to it during the wearing thereof” is more strongly required because the loads applied to the disposable diaper are heavier and the amount excreted per one time is larger than in the case of disposable diapers for infants. In addition, reduction in thickness like underwear tends to be strongly required for the performance required for disposable diapers. A typical solution to the requirement is a method to reduce the amount of pulp in the absorber of the disposable diaper and use a large amount of water-absorbing resin (for example, refer to Patent Document 24). In such an absorber, the water-absorbing resin is in a dense state and liable to cause gel blocking. As a result, conventional water-absorbing resins are inconveniently liable to cause reduction in water absorption capacity under pressure. On the other hand, when crosslinking density is increased to ensure the water absorption capacity under pressure, there may be a problem of reduction in water absorption capacity under no pressure.
[Patent Document 1] Japanese Patent Laid-Open No. 55-84304
[Patent Document 2] Japanese Patent Publication No. 49-43395
[Patent Document 3] Japanese Patent Laid-Open No. 51-125468
[Patent Document 4] Japanese Patent Laid-Open No. 52-14689
[Patent Document 5] Japanese Patent Publication No. 53-15959
[Patent Document 6] Japanese Patent Laid-Open No. 58-180233
[Patent Document 7] Japanese Patent Laid-Open No. 61-16903
[Patent Document 8] Japanese Patent Laid-Open No. 59-189103
[Patent Document 9] Japanese Patent Laid-Open No. 52-117393
[Patent Document 10] Japanese Patent Laid-Open No. 51-136588
[Patent Document 11] Japanese Patent. Laid-Open No. 61-257235 [Patent Document 12] Japanese Patent Laid-Open No. 61-211305 [Patent Document 13] Japanese Patent Laid-Open No. 61-252212 [Patent Document 14] Japanese Patent Laid-Open No. 61-264006 [Patent Document 15] Japanese Patent Laid-Open No. 60-163956 [Patent Document 16] Japanese Patent Laid-Open No. 60-255814[Patent Document 17] Japanese Patent Laid-Open No. 1-292004[Patent Document 18] Japanese Patent Laid-Open No. 2-153903 [Patent Document 19] National Publication of International Patent Application No. 1996-508517[Patent Document 20] Japanese Patent Laid-Open No. 6-122707 [Patent Document 21] Japanese Patent Laid-Open No. 6-122708 [Patent Document 22] Japanese Patent Laid-Open No. 2004-315816 [Patent Document 23] National Publication of International Patent Application No. 1996-509521[Patent Document 24] Japanese Patent Laid-Open No. 06-057010